WPAN has two operation modes. One is a beacon-enabled mode, and the other is a non beacon-enabled mode. In the non beacon-enabled mode, all the nodes in the network contend for channels using an unslotted Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) algorithm. The advantage of the non beacon-enabled mode lies in its self-organization. However, such mode does not provide a time guarantee and Quality of Service (QoS) guarantee.
In the beacon-enabled mode, the network may periodically transmit a superframe to organize communication. In a superframe manner, non-contention slot is allocated in a superframe to conduct communication, a real-time communication and QoS can be achieved. Compared with the non beacon-enabled mode, the beacon-enabled mode can provide a better real-time communication and QoS guarantee.
Superframe structure is specified in a related standard. FIG. 1 illustrates an existing superframe structure. As shown in FIG. 1, the transmission interval between two consecutive beacon frames is denoted as Beacon Interval (BI). A superframe includes a beacon, an active period and an inactive period. The active period includes a beacon frame transmission period, a contention access period (CAP), and a contention free period (CFP). In the inactive period, nodes do not transmit data. They enter sleep mode to save energy. The nodes mentioned herein include energy restricted nodes and energy unrestricted nodes. Corresponding nodes are energy restricted devices and energy unrestricted devices.
The active period of a superframe is also referred to as a superframe duration (SD) which is divided into 16 time slots with equal length, labeled as 1-15 in FIG. 1. The parameters such as the duration of each slot and number of slots in the CAP are predetermined in the network and these parameters are broadcasted to all the nodes in the network in a beacon frame transmission period at the beginning of a superframe.
BI and SD are associated with Beacon Order and Superframe Order respectively. The calculation formulas are expressed as in equation (1) and equation (2).BI=αBaseSuperframeDuration×2BO  (1)SD=αBaseSuperframeDuration×2SO  (2)
where parameter aBaseSuperframeDuration is a minimum length of a superframe when SO=0. Parameter aBaseSuperframeDuration specified in the standard includes 960 symbols, where 1 symbol=4 bits.
As specified in the standard, in a CAP period, the nodes contend for channel to transmit data with a slotted CSMA/CA algorithm. However, if a Guaranteed Time Slots (GTS) mechanism is used, in the CAP, the node may transmit a request for allocating GTS to the network. After GTS is acquired successfully, the node may transmit data directly in the acquired GTS, which eliminates the need for channel contention with a CSMA/CA algorithm. The minimum length aMinCAPLength for a CAP is 440 symbols. However, if GTS is used, the length of the CAP can be less than the minimum length.
In one superframe, only at most 7 GTSes can be allocated in one superframe. Each GTS is constituted by several slots.
Currently, the management of a superframe includes GTS allocation, cancellation of the allocated GTS, readjustment of the position of the allocated GTS, etc.
To request for allocating GTS, a node may transmit a GTS request to a coordinator. The GTS request may carry GTS characteristics such as the length and the direction of the GTS requested, etc. Usually, the coordinator may reply an acknowledgement frame to the node after receiving the GTS request. After the coordinator receives the GTS request, the coordinator may first check if a current superframe has enough capacity according to the remaining length of CAP and the length of the requested GTS. If the capacity has not reached the maximum number of GTSes or the length of the requested GTS does not render the length of CAP to be less than the minimum length, which means that the superframe has enough capacity, then the coordinator may allocate GTS to the node according to the request. Usually, the coordinator allocates GTS according to a first-come-first-served policy.
The allocated GTS can be cancelled or the position of the allocated GTS can be adjusted, etc. FIG. 2a, 2b 2c is a first schematic, a second schematic and a third schematic of existing GTS management, respectively. The inactive period in the superframe is not shown in FIG. 2a-2c. As shown in FIG. 2a, suppose three allocated GTSes are GTS1, GTS2, and GTS3. GTS1 occupies slot 14 and slot 15. GTS2 occupies slot 10 to slot 13. GTS3 occupied slot 8 and slot 9.
If GTS2 is cancelled, an unoccupied slot may appear in the superframe, as shown in the shadow portion of FIG. 2b. As can be seen, the superframe becomes fragmental which might cause the waste of resources. To tackle this problem, GTS position needs to be readjusted, i.e., moves GTS3 right close to the GTS1 such that the GTS continuity is guaranteed.
Compared with the non beacon-enabled mode, although the beacon-enabled mode in a superframe fashion may improve the real-time of the transmission and QoS in a certain degree, a node is not distinguished as an energy restricted node or an energy unrestricted node. The current practice in management of superframe may assure a low power consumption of the energy restricted node but may not be able to improve and guarantee the QoS of the energy unrestricted node. The management of superframe is not flexible, which hardly meets the requirement of transmission quality of a service requiring for a demanding real-time performance. Moreover, the existing practice in management of superframe does not consider fully and make full usage of the inactive period in the superframe, thereby decreasing significantly the network throughput.